1. Field of the Invention
The present invention relates generally to communications systems and particularly to power control in a code division multiple access communication system.
2. Description of Related Art
Because the radio frequency (RF) spectrum is limited, the government, more particularly, the Federal Communications Commission (FCC), governs the use of the radio frequency spectrum. This regulation includes deciding frequency band allocation among the various industries. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. Accordingly, the assigned spectrums must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.
Because the number and size of frequency bands are limited, multiple access modulation techniques are continuously being developed and improved to improve efficiency and capacity and to maximize use of the allocated RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. CDMA modulation techniques are becoming popular because they enable more users to communicate at a given time. A spread spectrum system uses a modulation technique that distributes the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required for transmitting the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wideband spreading code. Using this technique a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition. In addition, spreading the signals over a large bandwidth allows system robustness against frequency selective interference, as the effect of the de-spreading process is to effectively dissipate the interference power over the entire bandwidth.
In a CDMA telephone system, multiple signals are transmitted at the same frequency. A particular receiver then determines which signal is intended for that receiver by the unique spreading code in the signal. The signals at that frequency without the particular spreading code intended for that particular receiver appear as noise to the receiver and are ignored. Because of this, it is desirable in CDMA systems to transmit at a minimum power level. Thus, CDMA systems typically employ power control algorithms to reduce the power transmission levels. By minimizing power transmission levels, interference to other signals is reduced and network capacity is maximized.
New generation CDMA communication networks are being formed to facilitate the transmission of large amounts of data on an as needed basis. Accordingly, a fundamental channel set is defined for transmitting on going communications between the base station transceiver systems and the mobile stations. Specifically, the fundamental channel can be used for control information in the form of messages, voice transmission, and even data transmission. Additionally, supplemental channels are being defined to transmit large amounts of data to and from a mobile station for use as needed. The supplemental channel can be configured at a number of different data rates, depending on the channel conditions, user profile, etc. The fundamental channel only works at one low data rate.
In next generation CDMA systems the supplemental channel is used for the transmission of large amounts of data on a non-periodic and predictable basis. The data rate is negotiated between the terminal and base station according to various radio resource management algorithms. Once the data rate is negotiated and the supplemental channel is set up, the data transfer commences.
Frame erasures that occur during the data transfers are handled by a combination of power control algorithms and a Radio Link Protocol process (RLP). The power control algorithm(s) ensures that the transmission power is sufficient for the current channel conditions to meet a specified frame error rate target. The RLP procedure uses a form of Automatic repeat request (ARQ) to facilitate the retransmission of bad frames.
As a part of the aforementioned data rate negotiations, the time for which the supplemental channel (SCH) is to remain active is also negotiated. The SCH active timer can be set to infinity or some other finite value (exact choice of parameters is based on the International Standards Organization (ISO) TIA IS2000 standard). As the mobile is transmitting data to the base station(s), it may run out of data to send, either because there is no more data to be sent or because there is some delay within the network.
When the system enters a state in which there is no data to send, an active timer starts, and will be reset when data transmission resumes, otherwise once the timer expires, the SCH data call is moved from the active state to the dormant state, where all resources are torn down. When there is no data to send on the supplemental channel, the terminal sends NULL frames to maintain the power control loop stability, but at the expense of wasted power. Alternatively the SCH can go into DTX (discontinuous transmission) mode, where nothing is sent.
A method or algorithm is needed, therefore, to detect when the SCH is in DTX mode to prevent instability in the power control loop and/or the RLP process. If the network cannot differentiate between the DTX frame (i.e. no frame) or an Erasure (i.e. bad frame) then the system could feedback to the terminal asking to increase power, and also increase various power control thresholds in the network, ultimately sacrificing capacity. Normally, in reverse link power control, the SCH power is controlled indirectly from the FCH. Typically, the SCH power to FCH power ratio is kept fixed. If FCH transmissions are subjected to an erasure, then the power of the FCH is increased thereby also increasing the power of the SCH. Generally, for example, the ratio of SCH to FCH power is kept such that if the FCH is operating at about a 1% frame error rate target while the SCH operates at about 5% frame error rate. Thus, if power for the FCH is increased, so to is the power of the SCH in the IS2000 Standard. Without DTX detection, however, this process is problematic.
An additional advantage of operating in the DTX mode lies on the fact that the mobile station can conserve power by using less power. This results in longer battery life as well as lower interference to other users. Assuming that the mobile station transmitting over a supplemental channel operates in the DTX mode, the base station cannot, under current designs, distinguish between an SCH frame erasure and a DTX mode of operation. The inability to make this distinction has an adverse affect on the higher radio link protocol (RLP) layers. For example, set points (threshold values for making power increase/decrease requests) used by a base station in its power control algorithm may be raised in response to a determination that a frame has been erased. Additionally, various algorithm enhancements, such as reverse link power control, may perform poorly if a base station is not able to detect the DTX mode of operation by the mobile station.
By way of example, if a base station is not aware that a mobile station is operating in a DTX mode of operation, the base station power control algorithms are likely to conclude that an erasure has occurred and that the mobile station should increase its transmission power levels. Accordingly, the base station generates, in this situation, power control commands to quickly increase the mobile station power transmission levels to receive a signal. Thereafter, because the signal to noise ratios for future further received signals will be above a specified threshold, the mobile station will be instructed to decrease power transmission levels.
In typical designs, however, a series of much smaller steps are used to decrease the transmission power level. The effect of this whole process is that, for a period of time, the mobile station transmits at a higher power level than is required thereby interfering with other carriers.
What is needed, therefore, is a system that enables a base station to determine when a mobile station is either transmitting a Null set or is operating in a DTX mode of operation.
The present system and method of use comprises a system that solves the aforementioned problems by, in part, monitoring a frame quality metric for both the fundamental channel and the supplemental channel. When a specified value(s) of frame quality metrics for the supplemental channel and fundamental channel is detected, the invention includes determining if an erasure has occurred or if the mobile is in a DTX mode.
More specifically, if each of the base stations receiving fundamental and supplemental channel data from a mobile station determine that the specified FER rate(s) have been observed, the invention includes an additional step of comparing an estimated delta offset value to an actual delta offset value. If the difference between the offset values is greater than a defined number, then the base station can conclude that the mobile station is operating in a DTX mode of operation. If the difference between the estimated delta offset value and the calculated (actual) delta offset value is less than or equal to the specified number then the base station can properly determine that an erasure of the signal has occurred.
The measured (actual) offset value is calculated by first calculating a signal to noise ratio for the pilot channel and then calculating a signal to noise ratio for the supplemental channel and then by dividing the calculated ratio for the pilot channel by the calculated ratio for the supplemental channel.